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Polish Journal of Environmental Studies Vol. 8, No. 5 (1999), 313-318
The Spectrophotometry Determination of Chelate
Complex: L-Ascorbic Acid with Cuprum (II) and
Mercury (II) in Alkaline Solution
E. Kleszczewska
University in Białystok, Institute of Chemistry, J. Piłsudskiego 11/4, 15-443 Białystok, Poland
Received 20 March,1999
Accepted 10 June, 1999
Abstract
The aim of this paper is to present the chemical behaviour of cuprum (II) and mercury (II) in chelate
complexes with L-ascorbic acid in alkaline solution. The results were interpreted by the spectroscopy method.
Composition and total stability constants of the complexes were determined by the Jacymirski method. In
solution in with pH = 9, H2 Asc forms complexes of the type [Me(Asc)2]-2; Me : L = 1:2. The complexes are of
medium stability.
Keywords: L-ascorbic acid, cuprum (II), mercury (II), spectrophotometry, chelate complex
Introduction
Our interest in L-ascorbic acid (H2Asc) comes, despite
its major biological role as vitamin C [1-4]. From that,
studies of this interaction with metal ions are limited.
According to literature, the principal mechanism of this
reaction is presented either as the oxidation of L-ascorbic
acid to L-dehydroascorbic acid (Asc), via the free radical
(HAsc*) from similarly to the reaction of H2Asc with
Cu(II) [5-8]:
mercury in living organisms. The present study comprises
investigation on ascorbinate complexes of Cu(II) and
Hg(II) in alkaline solution.
H2Asc + Cu(II) ↔ HAsc* + Cu(I) + H+
2HAsc* ↔ H2Asc + Asc
or by the formation of a complex [9, 10].
In recent years, particular attention has been paid to
studies on the complexation of important bioelements with
biologically active ligands [11-13]. The presence of the
dienol group (Fig. 1) in the molecule of the L-ascorbic acid
allows us to assume possible complexation of the compounds with metal ions [10, 12-14].
Few authors have dealt with metal ascorbinate, and therefore, these complexes are not yet completely known. Relatively great attention has been paid to ascorbinate complexes of transition metals [9, 13-15] and lanthanide series
[9]. This is associate probably with the role of cuprum and
Fig. 1. L-ascorbic acid structure.
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Kleszczewska E.
Experimental Procedures
Analar L-ascorbic acid manufactured by Fluka was
used. The basic solutions of metal salts were used in the
form of nitrates Cu(NO3)2 and Hg(NO3)2 analar grate of
Gliwice. The pH value of the solutions investigated was
adjusted with standard solutions of NaOH - Gliwice, prepared from analar reagents. Ionic strength was controlled
with NaNO3, analar grate Gliwice. L-ascorbic acid (vitamin
C), C6H8O6, H2Asc reagent:
- stock solution with the concentration of 1 • 10-2
mol/dm3 was obtained by dissolving 0.176 g of the substan
ce in a measuring flask of 100 cm3 capacity.
Copper (II) nitrate (V), Cu(NO3)2 • 3H2O reagent:
- stock solution with the concentration of 1 • 10-2
mol/dm3 was obtained by dissolving 0.242 g of the substan
ce in a measuring flask of 100 cm3 capacity.
Mercury(II) nitrate (V), Hg(NO3)2 • H2O reagent:
- stock solution with the concentration of 1 • 10-2
mol/dm3 was obtained by dissolving 0.363 g of the substan
ce in a measuring flask of 100 cm3 capacity.
UV absorption spectra within the range of 190-350 cm-1
of aqueous solution of L-ascorbic acid with the concentration of 1 • 10-4 mol/dm3 were completed.
The following procedure was used: 1 cm3 of the solution of L-ascorbic acid with the concentration of 1•10-2
mol/dm3 was added to a 100 cm3 measuring flask and then
it was made up to the mark with water.
UV absorption spectra of aqueous solution of L-dehydroascorbic acid within the range of 190-350 cm-1 were carried out.
The following procedure was used: 1 cm3 of the solution of L-ascorbic acid with the concentration of 1 • 10-2
mol/dm3 was added to a 100 cm3 measuring flask and was
then made up to the mark with water. Then, the obtained
solution was exposed to UV. After 24 hours the absorbency
of the solution against water as reference material was measured.
In order to examine the influence of the medium on
absorption of L-ascorbic acid, UV spectra within the range
of 190-350 cm-1 in solutions at different pH were done. The
examinations were done directly after blending and after 24
hours and exposure to UV.
The following procedure was used: 1 cm3 of the solution of L-ascorbic acid with the concentration of 1 • 10-2
Fig. 2a. Spectrophotometry curve of L-ascorbic acid pH = 7.
mol/dm3 was added to a 100 cm3 measuring flask and
then the solution of NaOH with proper concentration was
added in such quantity that, after making up to the mark,
pre-determined pH, e.g. pH = 9, could be obtained.
Solutions of NaOH or sodium base with suitable pH
were used as reference material.
UV-VIS absorption spectra of aqueous solutions of copper and mercury within the range of 190-900 cm-1 were
done to obtain the characteristics of copper and mercury.
The following procedure was used: in a 100 cm2 measuring flask the solutions of the examined compounds with
the concentration of 1 • 10-2 were prepared by dissolving an
appropriate weighed amount. Then, the absorbency of the
solution against water was measured.
In order to investigate the influence of the medium on
the shift of absorption maximum of examined metals,
UV-VIS absorption spectra of the examined compounds
within the range of 190-800 cm-1 in solutions with different
pH values were done.
The following procedure was used: in a 100 cm3 measuring flask the solution of the examined metal with the concentration of 1 • 10-2 was prepared. Then the solution of
sodium base with proper concentration was added. Suitable
solutions of NaOH or sodium base with proper pH were
used as reference material.
The solution containing L-ascorbic acid with lower concentration and sodium nitrate with concentrations equal to
the examined solution were used as reference material.
UV spectra of L-ascorbic acid, its complex with cuprum
(II) and mercury (II) and L-dehydroascorbic acid, in aqueous solution, were carried out with a CESIL INSTRUMENTS CE 8020 spectrophotometer.
Results and Discussion
I. Preliminary study
The mechanism of reaction L-ascorbic acid - metal is
dependant on the pH of the reacting solution. It is known
that the pH of solution strongly influences the spectrophotometric characteristic of L-ascorbic acid and metal ions [3, 4,
7]. In connection with this, the influence of pH on shape
and positions of absorption bands of L-ascorbic acid, L-dehydroascorbic acid and metal ions was examined at the
beginning of the work. For this purpose, the absorption
Fig. 2b. Spectrophotometry curve of L-ascorbic acid pH = 9.
The Spectrophotometry ...
315
Table l.The λmax values and molar absorption coefficients (ε) of
H2Asc solutions at different pH values.
spectra of aqueous solution of all reactants were recorded.
The results are assembled in Tables 1-3.
It was observed that aqueous solutions of L-ascorbic
acid with the concentration of 1 • 10-4 mol/dm3 show very
characteristic bands in UV spectrum, with maxims at 266
in neutral solution and 276 nm in alkaline solution (Table 1,
Figs. 2a and 2b).
After that, it was noted that L-dehydroascorbic acid
aqueous solution (at a concentration of 1 • 10-4 mol/dm3),
shows maximum absorption at λmax = 300 nm. Next, the
influence of the medium was examined. For that purpose
the UV spectra of L-dehydroascorbic acid in solutions at
different pH were done. Obtained λmax (Fig. 3) and molar
absorption coefficients are shown in Table 2.
The results of the examination show that pH has little
influence on the location of L-dehydroascorbic acid signals.
In Table 3 and Fig. 4 we show aqueous solution with
the concentration of 1 • 10-4 mol/dm3 of various nitrates
Cu(II) and Hg(II) at different pH.
Fig. 3. Spectrophotometry curve of L-ascorbic acid pH = 9.
Table 2. The λmax values and molar absorption coefficients (ε) of Asc
solutions at different pH values.
Table 3. The λmax values and molar absorption coefficients (ε) of
various nitrates Cu(II) and Hg(II) solutions at different pH values.
Fig. 4. Spectrophotometry curves of cuprum(II): a - pH = 7, b - pH = 9 and mercury(II): c - pH = 7, d - pH = 9.
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Kleszczewska E.
Table 4. Some exemplary stability constants calculated using the Jacymirski method (e.g. L-ascorbic acid -cuprum (II)).
Table 5. Some exemplary stability constants calculated with the Jacymirski method (e.g. L-ascorbic acid-mercury (II)).
Analyzing the absorption spectra of the above compounds at pH, one can conclude that the absorption maxima are practically equal at range 7-9.
II. The study of reaction of L-ascorbic acid - Cu(II) and
L-ascorbic acid - Hg(II).
In solution L-ascorbic acid behaves as a weak dibasic
acid, which dissociates in two steps. In the first step one
hydrogen ion is released:
H2Asc ↔ H+ + HascThe corresponding ionization constant can be expressed as:
Fig. 5. Dependence of function f1 on CL; a1 = -11.726 • 1010.
Hasc- ↔ H+ + Asc2(using concentrations instead of activities). The value of
the ionization constant for the stronger acid hydrogen has
been determined by several authors. Most reliable values
were obtained by Birch and Harris [7] (K 1 = 6.77 • 10 -5 ,
pK 1 = 4.17) and later by Ball [7]. In the text, Ball's data,
e.g. K1 = 6.2 • 10-5 (pK1 = 4.21) will be employed as the
ionization constant and its exponent, respectively, for the
first dissociation step.
In the second dissociation step a further hydrogen
ion is released. Birch and Harris obtained the value
K2 = 2.69 • 1 0 -1 2 ( p K 2 = 11.57).
The run of reaction L-ascorbic acid - metal ions was
followed spectrophotometrically using the following proce-
Fig. 6. Dependence of function f2 on CL; a2 = 3.06 • 1016
317
The Spectrophotometry ...
structure of chelate complex [15, 16] L-ascorbic acid : cuprum (II)
Fig. 7. Depencence of function ∆ε e on 1/[L].
Fig. 8. Dependence of function φ1 on 1/[L].
dure. The 89 ml portion of 1 • 10 -4 mol/dm 3 Cu(NO 3 ) 2
aqueous solution was placed into a 100 ml glass-tube.
Next, the 10 ml of 1 • 10 -2 mol/dm3 aqueous solution of
H2Asc were added. The pH of reacting solution was adjusted using 1 • 10 -2 mol/dm 3 NaOH. We have in solution
Asc2- ion, because the Asc-2 ion can only exist in alkaline
medium. Then, the mixture was shaken vigorously and was
recorded.
In alkaline medium at pH = 9 the following complexes
are formed:
- cuprum (II) and L-ascorbic acid with maximum absor
ption at 206 nm, and
- mercury (II) and L-ascorbic acid with maximum ab
sorption at 266 nm
The stability constants was calculated using the of Jacymirski method [17, 18] (Tables 4, 5 and Figs. 5-8).
The stability constants calculated by the Jacymirski method show in Table 6.
Table 6. The stability constant chelate complex.
From the spectrometric examination one may concluded
that in the alkaline medium this ratio is 1:2. We suggested
References
1. PODLEWSKI J. K., CHWALIBOGOWSKA-PODLEWSKA
A.: Leki wspolczesnej terapii 1998, Spilt Trading, Warszawa
1998.
2. ZIEMLANSKI S., BULHAK-JACHYMCZYK B., BUDZINSKA-TOPOLOWSKA: Normy zywieniowe dla ludnosci
w Polsce (energia, bialko, tluszcze, witaminy, skladniki mineralne), Zyw. Czlow. Metab., 21 (4), 303, 1994.
3. FRIDRICH W.: Vitamins, Walter der Gruyter, Berlin-New
York 1988.
4. Witaminy i Mikroelementy, Dzienne zapotrzebowanie, ograniczenia w stosowaniu, Pruszynski i S-ka, Warszawa 1997.
5. WITKOWSKA J., CZERWINSKA D., KIEPURSKI A., ROSZKOWSKI W.: Pierwiastki szkodliwe a zelazo, cynk
i miedz: interakcje w organizmie zwierzat i ludzi, Rocz. PZH,
1 (42), 15, 1991.
6. SAUBAERLICH H.: Vitamin Cand Immunity. IN: Parmacology of vitamin C (ED: H. Saubaerlich). Annau. Rev. Nutr.,
14, 382, 1994.
7. KLESZCZEWSKA E., PUZANOWSKA-TARASIEWICZ H.:
L-ascorbic acid -occurrence, properties, method of determina
tion and application in chemical analysis, Rocz. Akad. Med.,
38 (2), 248, 1993.
8. RUTKOWSKI M., GRZEGORCZYK K.: Witaminy o dzialaniu antyoksydacjnym -ogolna charakterystyka. Czesc III: Witamina C, Farm. Pol., 55 (2), 74-79, 1999.
9. KLESZCZEWSKA E., KOZAK1EWICZ H.: The Analysis of
Cuprum and Mercury Combinations with L-ascorbic Acid
Kleszczewska E.
318
10.
11.
12.
13.
14.
Dependent on the Level of Metal Oxidation, Toxicol. Lett., 95
(1), 130, 1998.
SEIB P. A., TOLBERT B. M.: Ascorbic Acid: Chemistry,
Metabolism and Uses, Washington D. C. 1982.
MASLOWSKA J., OWCZAREK A.: Studies on Ascorbinate
complexes of Metal Ions of Berylium Grup by the Method of
Potentiometric Surfaces, Polish J. Chem., 57, 719, 1983.
MASLOWSKA J., OWCZAREK A.: Potentiometric Studies
on Mixed Complexes of Alkaline Earth Group Metals with
Ascorbic and Tartaric Acid, Polish J. Chem., 62, 75, 1988.
KUTSKY R. J.: Hanbook of vitamins and hormons, Van Nostrand Reihold Company, New York 1973.
KLESZCZEWSKA E.: The Spectrophotometry Determination
of Chelate Complex: L-ascorbic Acid with 10d Element
15.
16.
17.
18.
(Zn(II), Cd(II) and Hg(II)), Polish J. Environ. Stud., 6, 84,
1997.
KLESZCZEWSKA E., MONIUSZKO-JAKONIUK J., KLESZCZEWSKI T.: Theoretical Evidence of H-Bridges for the
Complexes of L-Ascorbic Acid and Cadmium (II), Polish J.
Environ. Stud., 5 (6), 41, 1996.
KLESZCZEWSKA E., KLESZCZEWSKI T.: The Optimisa
tion of the Geometry of the Octaedral Complex of L-Ascorbic
Acid and Metal (II) Using Semi-emporical CNDO Calculation
with Minimization Based on Polak-Ribiere'a Method - in
press.
JACYMIRSKI K.B., WASILIEW W.P., Konstanty niestojkosti kompleksnych sojedinienij, Moskwa, 1959.
JACYMIRSKI K.B., Zurnal Fiz. Chem. 1, 2306, 1956.